Bayern coach Thomas Tuchel baffled by shock home loss to RB Leipzig
Bayern remain in top spot on 68 points but Dortmund, on 67, are in action at Augsburg on Sunday and a win would see the Ruhr valley club go into next week's season finale in the driving seat.
"When you play so far below your level and you keep dropping that level then you will struggle to win games and when you struggle to win games it is just not enough," Tuchel said.
"I have no explanation how something like that could happen. It is inexplicable. We stopped moving, stopped helping, stopped showing courage."
Bayern looked to be cruising to an easy win after a good start with several chances and a lead through Serge Gnabry (27) in the 25th minute.
But they instantly eased off and Leipzig came back with three goals in the second half to earn a maiden win in Munich and leave the title race wide open.
"The first 30 minutes were good, we could have been 2-0 up or more," Tuchel said. "But then we started being sloppy and sluggish. In the second half for no reason we were extremely sloppy. I have no explanation."
Tuchel, who won the Champions League with Chelsea in 2021, took over in late March after the surprise sacking of Julian Nagelsmann (35) with club bosses at the time saying season goals were at risk and a change was necessary.
But the Bavarians have since crashed out of the German Cup and Champions League in the quarter-finals and now risk missing out on an 11th straight league crown.
"I am disappointed because we can do it so much better," Tuchel said. "We had the game in our hand and gave it away.
"It is still not over," Tuchel said of their title chances. "Let's see what happens on the last matchday. Obviously we wanted things differently."